This invention relates generally to voltage-controlled tuning systems for tuning a television receiver to a desired channel and specifically to a single tuner circuit for tuning to all VHF/UHF/CATV channels without utilizing an accessory band converter.
The tuner in a typical electronically tuned television receiver includes the following four tunable stages: an antenna input, or RF, amplifier stage, an RF output amplifier stage, a mixer input stage, and a tuned oscillator stage. Each stage is tuned to resonate at a given frequency representing the desired channel. All stages are tuned simultaneously and in combination form a multi-stage tunable circuit.
A voltage-controlled tuner includes, in each of the aforementioned tunable resonant circuits, a variable reactance device that tunes the circuit to different channels in response to changes in amplitude of a DC control, or tuning, voltage applied to the device. Usually, each of these devices takes the form of a voltage-controlled variable capacitance diode, commonly called a varacter or varicap diode, the capacitance of which is determined by the magnitude of the applied tuning voltage. As the tuner is actuated to different positions, the applied voltage level varies in order to change the resonant frequency of each tunable circuit to effect tuning to different channels.
When more than one frequency band is to be covered by the tuner, bandswitching may be achieved by using a variety of different approaches, while permitting the varactor diodes to be common to and used in each band. For example, for each band a different inductance coil may be mechanically switched into each tunable circuit. As another example, each tunable circuit may comprise a sectionalized inductance coil with different sections being shorted out by mechanical switches for tuning to different bands. To avoid the translation of RF signals through the mechanical switches in the last example, electronic switching or gating devices may be used. Typically used in such applications are diodes which are capable of being turned ON by one magnitude-polarity condition of the switching voltage and turned OFF in response to another magnitude-polarity condition.
An approach employing this varactor-controlled tuning technique in combination with diode band-switching is disclosed in U.S. Pat. No. 3,646,450. Therein is described a system capable of electronically switching between low (channels 2-6) and high (channels 7-13) VHF tuning bands. This is accomplished by selectively switching an inductive coil in and out of each of the four tunable stages by applying one of two input voltages to the diode. Individual channel tuning is accomplished by varying the voltage to a varactor in the tuned circuit thereby adjusting its operating frequency to a value representing the desired channel. This variable voltage, channel tuning approach is limited to application in the VHF and UHF broadcast bands.
An approach for tuning a television receiver to VHF, UHF and CATV channels is disclosed in U.S. Pat. No. 3,931,578 wherein is described a multiple frequency band receiver tuner system adapted to operate in a double frequency conversion mode and utilizing a single non-bandswitched local oscillator. In this approach frequency multiplying and/or frequency dividing stages are connected to the output of the local oscillator to provide the UHF and VHF-CATV mixer circuits with appropriate local oscillator frequencies to develop the desired IF frequencies. The approach of the '578 patent primarily involves the dual use of a tuned VHF RF amplifier as a tunable first IF amplifier stage in a UHF double conversion receiver which provides for the reception of multiple frequency bands. In addition, frequency multiplication of the single local oscillator output is utilized for reception of UHF while frequency division of the oscillator output is utilized for reception of the VHF low band.
Another multi-band tuner utilizing varactor diodes is disclosed in U.S. Pat. No. 3,564,423 wherein the varactor diodes are utilized not only for tuning in individual channels but also for bandswitching. In that approach de-coupling between the separate resonant circuits used in each frequency range is achieved by means of a plurality of bridge circuits with associated elements, the neutral branch of which includes the resonant circuit for the subsequently lower frequency range. Thus, resonant circuit de-coupling and improved tuning of the individual resonant circuits resulting therefrom is achieved in spite of the fact that the resonant circuits are always connected together. The '423 approach, while intended to avoid the undesirable effects of mechanical switching, fails to take advantage of current electronic switching technology and, as a result, suffers from undue complexity. U.S. Pat. No. 4,115,737 describes a system for receiving signals broadcast on two spaced bands each divided into several channels and makes use of two fixed band pass filters, one for each of the two frequency bands. U.S. Pat. No. 3,628,152 discloses a varactor diode tuning system intended to maintain a constant bandwidth as the system is tuned throughout its frequency range to avoid problems to bandwidth variation caused by changes in circuit capacitance. This tuning circuit is limited in application to tuning over the VHF and UHF broadcast bands.
The present invention represents a significant advance in the art because of its capability to tune to all VHF, UHF and CATV channels without utilizing a frequency converter, as required in currently available tuning systems. The present invention not only incorporates the advantages of electronic bandswitching but also permits easy inductor coil adjustment in facilitating tuning circuit alignment with a minimum number of components in a low cost tuner.